1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for processing seismic data.
2. Discussion of the Background
During the past years, the interest in developing new oil and gas production fields has dramatically increased. However, the availability of land-based production fields is limited. Thus, the industry has now extended drilling to offshore locations, which appear to hold a vast amount of fossil fuel. Offshore drilling is an expensive process. Thus, those engaged in such a costly undertaking invest substantially in geophysical surveys in order to more accurately decide where to drill in order to avoid a dry well.
Marine seismic data acquisition and processing generate a profile (image) of the geophysical structure (subsurface) under the seafloor. While this profile does not provide an accurate location for the oil and gas, it suggests, to those trained in the field, the presence or absence of oil and/or gas. Thus, providing a high resolution image of the subsurface is an ongoing process for the exploration of natural resources, including, among others, oil and/or gas.
During a seismic gathering process, as shown in FIG. 1, a vessel 10 drags an array of acoustic detectors 12. Plural acoustic detectors 12 are disposed along a cable 14. Cable 14 together with its corresponding detectors 12 are sometimes referred to, by those skilled in the art, as a streamer 16. The vessel 10 may tow plural streamers 16 at the same time. The streamers may be disposed horizontally, i.e., lying at a constant depth z1 relative to a surface 18 of the ocean. Also, the plural streamers 16 may form a constant angle (i.e., the streamers may be slanted) with respect to the surface of the ocean as disclosed in U.S. Pat. No. 4,992,992, the entire content of which is incorporated herein by reference. FIG. 2 shows such a configuration in which all the detectors 12 are provided along a slanted straight line 14 making a constant angle α with a reference horizontal line 30.
With reference to FIG. 1, the vessel 10 also drags a sound source 20 configured to generate an acoustic wave 22a. The acoustic wave 22a propagates downward and penetrates the seafloor 24, eventually being reflected by a reflecting structure 26 (reflector R). The reflected acoustic wave 22b propagates upwardly and is detected by detector 12. For simplicity, FIG. 1 shows only two paths 22a corresponding to the acoustic wave. However, the acoustic wave emitted by the source 20 may be substantially a spherical wave, e.g., it propagates in all directions starting from the source 20. Parts of the reflected acoustic wave 22b (primary) are recorded by the various detectors 12 (the recorded signals are called traces) while parts of the reflected wave 22c pass the detectors 12 and arrive at the water surface 18. Since the interface between the water and air is well approximated as a quasi-perfect reflector (i.e., the water surface acts as a mirror for the acoustic waves), the reflected wave 22c travels back towards the detector 12 as shown by wave 22d in FIG. 1. Wave 22d is normally referred to as a ghost wave because this wave is due to a spurious reflection. The ghosts are also recorded by the detector 12, but with a reverse polarity and a time lag relative to the primary wave 22b. The degenerative effect that ghost waves have on bandwidth and resolution of seismic measurements are known. In essence, interference between primary and ghost arrivals causes, among other problems, notches, or gaps, in the frequency content of the data recorded by the detectors.
The traces may be used to determine the subsurface (i.e., earth structure below surface 24) and to determine the position and presence of reflectors 26. However, the ghosts disturb the accuracy of the final image of the subsurface and for at least this reason, various methods exist for removing the ghosts, i.e., deghosting, from the results of a seismic analysis. Further, the actual measurements need to be processed for obtaining the correct position of the various parts (reflectors) of the subsurface. Such a processing method is the migration.
The migration ignores the presence of ghosts, i.e., it assumes that the ghosts are not present. However, a real migration cannot be based on this assumption. For this reason, the ghosts need to be removed, mathematically, before applying the migration process.
The ghost information may also be used to determine a final image of the subsurface. When using the ghost information, the primaries are imperfectly aligned while the ghosts are aligned and thus, the ghost information is determining the positions of the reflectors. For this reason, this processing is called in the art mirror migration and this process is described, for example, in: “Facilitating technologies for permanently instrumented oil fields”, Ebrom et al., The Leading Edge, Vol. 19, No 3, pp. 282-285, March 2000, the entire content of which is incorporated herein by reference. It is noted that during the mirror migration, the primary information is not used, which is the opposite of the migration process.
Based on a first final image produced by the migration process and a second final image produced by the mirror migration, a final combined image may be obtained by adding the two images together. In other words, FIG. 3 illustrates a traditional processing of seismic data. In step 40, seismic data are collected by firing the sound source and recording the received signals at the detectors. In step 42, the primaries are aligned which results in the ghosts being filtered out or reduced. Thus, mainly the primaries are used to migrate the data from the apparent position to the real position. Based on the results of the migration step, a final image A of the subsurface is generated in step 44. This final image may be used by those interested as is.
An alternative path for generating a final image is to use the mirror migration algorithm in step 46. In this step the ghosts are aligned which results in the primaries being filtered out or reduced. Thus, mainly the ghosts are used to migrate the data in the mirror migration. A different final image B is generated in step 48. This final image may be used by those interested as is. It is noted that each of steps 42 and 46 are processor intensive as a large number of equations have to be solved for taking into account the primaries or the ghosts. A better final image C may be obtained in step 50 if the final image A is added to final image B.
The above processing was discussed assuming that the detectors are provided at the same depth relative to the surface of the water. However, there are situations when the detectors are provided on slanted streamers, i.e., each detector has its own depth. For these situations, a supplementary step 52 may be necessary as shown in FIG. 3. This step is called in the art “datuming.” Datuming is a processing method in which, using the data from N seismic detectors provided at positions (xn, zn), where n=1, . . . N and N is a natural number, a synthesis is made of the data that would have been recorded by the seismic detectors if they had been placed at the same horizontal positions xn but at the same constant reference depth z0 for all the seismic detectors.
Datuming is one dimensional (1D), see for example, U.S. Pat. No. 4,353,121 (the entire content of which is incorporated herein by reference) when it is assumed that the seismic waves propagate vertically. In this case, the process is limited to applying a static shift to each time recorded by a given seismic detector, this static shift corresponding to the time of vertical propagation between the real depth zn of the n detector and the reference depth z0.
However, the toll imposed by running, for example, the inverse propagation equations, twice, once for the primary in the migration process and once for the ghosts in the mirror migration process is large in terms of the time and computer power necessary to process the data. For understanding the computational power involved in the seismic data processing, it is noted that the migration is a process that can occupy for several weeks several tens of racks of computers, each rack including hundreds of processors.
Accordingly, it would be desirable to provide systems and methods that avoid the afore-described problems and drawbacks, e.g., shorten the amount of time necessary to produce the final image.